The present invention relates to a method of manufacturing molded goods using paper or a pulp sheet as material.
It is intended to prevent the generation of cracks in the molded goods particularly in the case of forming a plurality of molded goods at once by corrugating the paper or pulp sheet in cross section in advance before molding and then subjecting the corrugated paper or pulp sheet to heating and pressing.
Food tray or cushioning media for packages made of plastics such as foamed plastics or vinyl chloride resin are used world-wide.
Molded goods of plastics have many advantages including low cost, mass productivity, light weight, elegant appearance and the like.
Plastic products which are now under investigation and development with respect to the disposal and re-use thereof, however, have still many unsolved problems and no practical solution has been reached yet.
Accordingly, the plastic products of this kind cause a great environmental problem in the world.
As a result, the development of substitutes for the plastic products is demanded.
Among them, a pulp molding method is known as a technology developed in this industrial field.
This method, as well known, makes use of waste-paper as material, which is molded and died in various shapes depending on its use.
The processes involved in this manufacturing method can be roughly classified into a molding process and drying process.
Explaining a general manufacturing process, the molding process comprises the steps of attaching a metallic forming die to the surface of a cylindrical drum, covering the surface of the die with a metal net, and soaking the drum in pulp and producing a vacuum inside the drum to laminate the pulp material on the surface of the die so as to obtain a pulp molded good which is the same as the die in shape.
Thereafter the molded good which is still in a wet state is subjected to the drying process to obtain a final product.
Since the product has an advantage of reusing the material or being easily incinerated, it is used in some industrial fields.
For example, it is used for a darkish cushioning medium to pack eggs therein or a cushioning medium to transport vegetables and fruits therein as often seen in supermarkets.
This manufacturing method, however, has problems such as taking a comparatively long time in manufacturing because of the drying process indispensable thereto, being limited in thickness of product (about 1 to 2 mm) because of the method of laminating pulp on the surface of the die, being not rigid, being inferior in waterproofness, being inferior in appearance and requiring a forming die having a complicated shape and moreover being very expensive in manufacturing the forming die, so that it is limited in use and consequently has not come into wide use.
Besides the pulp molding method, there is an invention disclosed in Japanese Patent Publication No. 1114380.
It is a method of manufacturing molded goods employing as the material a sheet of paper impregnated with resin powders therein, wherein the molded goods are obtained by subjecting corrugated molding paper which has been corrugated by a corrugator to heating and pressing.
The resulting product is excellent in strength and is advantageous compared with the molded good made of pulp.
Particularly in the case of using paper as the material, the paper cannot be prevented from being broken or cracked when it is subjected to heating and pressing since the paper is inferior in ductility and malleability as is commonly known.
Accordingly, a corrugated portion is provided in the material before molding according to the aforementioned invention to shrink the material, thereby providing the ductility and malleability required for molding so as to make the molding feasible.
This method, however, still has the following problems.
Since it uses as its material molding paper which is made from pulp solution impregnated with heat-hardening resin powders and cannot use paper on the market as it is, it becomes expensive in material cost causing an economical drawback.
Moreover, the heat-hardening resin powders mixed therein cause a problem in incineration.
Although the specification teaches that there occurs no problem in forming a plurality of molded goods, cracks were generated in molded goods in the experiment of the present inventor.
The prior art provides a so-called "relief` at a portion of the forming die to prevent the generation of cracks in the molded goods in manufacturing the same.
As a result, it caused the generation of wrinkles at the portion of the molded good corresponding to the relief of the forming die.
Moreover, the drawing depth was limited at most to that of a lunch box or the like and it was impossible to manufacture a cushioning medium having a complicated form for an industrial package (e.g. a cushioning medium for an acoustic device inside a package).
The present invention aims to solve the aforementioned problems of the prior art.
It is a first object of the present invention to obtain crackless molded goods in manufacturing a plurality of molded goods at one time.
It is a second object of the present invention to obtain molded good which are easily burnable and re-usable.
It is a third object of the present invention to enable the manufacturing of molded goods having complex shapes and deep drawings (i.e. deep cross-sectional shapes which, to be formed by press molding, must be deeply drawn).
Moreover, other advantages and features of the present invention will become clear by the following description with reference to the drawings.